Four wheel drive vehicles have become a major segment of the U.S. automotive market. With the advent of rising fuel prices, the pressure to improve the fuel efficiency of four wheel drive vehicles is more intense than ever.
It is contemplated that at least one reason for lower fuel efficiency of four wheel drive vehicles relates to the additional mass carried to make the vehicle capable of four wheel drive. Furthermore, many drive train systems are designed by the manufacturer to size the primary and secondary axle components according to the peak torque transmittable by the drive train rather than the peak torque required at the wheel to satisfy the vehicle's operational needs. The mass of such over-designed components may be greater than necessary.
For example, many transfer cases include mechanisms such as dog clutches or synchronizers to engage the four wheel drive system. In these systems, the front and rear driveline become rigidly linked by the splines of meeting components. No mechanical fuse or torque limiting device is present. Accordingly, a potential exists for spike loads induced in the drive line to pass through these mechanical components. In some cases, the spike loads may be twice the expected amount of torque required to slip a tire on asphalt. Components not sized properly may fail leaving the vehicle no longer drivable. While the spike loads may seldom occur, it may be necessary to design for them. Therefore, many existing designs are currently equipped with oversized components. These heavier systems provide a relatively low overall efficiency.
Electronic or hydraulic active torque transfer systems may effectively act as torque limiters to limit the torque transferred to the secondary axle. However, these devices most likely include relatively expensive and complicated actuation systems required to apply a clutch force as well as an electronic control module for properly sensing vehicle conditions and applying the clutch force appropriately. The cost associated with the active systems may be prohibitive. Accordingly, a need may exist to provide an automotive drive train solution providing improved fuel economy and lower overall four wheel drive system costs.